Initial defence responses in sapwood of Eucalyptus nitens (Maiden) following wounding and fungal inoculation

Abstract

Xylem defence responses occurring in pot-grown Eucalyptus nitens(Maiden) saplings were analysed within the first few weeks following stem wounding and inoculation with the non-aggressive decay fungus Ganoderma adspersum(Schulz). The fungus colonized the dead xylem cells up to 5mm above and below the wound after 30 days. Evidence of xylem cell death and discolouration was detectable within 12h and vessel tyloses were present after 3 days. Concentration of tetra-galloylglucose increased by 3.5-fold within 24h, while the trend for total phenols was a slow increase reaching a maximum after 21 days. Suberin was not detected. Early accumulation of water content has been proposed as a defence component of host-pathogen interactions in woody angiosperm xylem, but this was not obvious for E. nitens. Nuclear magnetic resonance imaging (NMR imaging or MRI) was used to measure small-scale distributions in moisture content around the inoculated stem wounds in intact E. nitens saplings. Some moisture accumulation was detectable at lesion margins of wounds analysed after 3 weeks. However, this was not detectable by gravimetric measurements. This increase in moisture content may be a consequence of wound physiology rather than an active defence response.